diff-highlight: add multi-byte tests

diff-highlight: add multi-byte tests

Now that we have a test suite for diff highlight, we can
show off the improvements from 8d00662 (diff-highlight: do
not split multibyte characters, 2015-04-03).

While we're at it, we can also add another case that
_doesn't_ work: combining code points are treated as their
own unit, which means that we may stick colors between them
and the character they are modifying (with the result that
the color is not shown in an xterm, though it's possible
that other terminals err the other way, and show the color
but not the accent).  There's no fix here, but let's
document it as a failure.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>